Sister Leonia, a Polish nun of the Sisters of Mercy of St Charles Borromeo, began work there in 1992. A trained nurse, she became concerned that the official government statistics on the incidence of Aids underestimated the scale of the problem in Zambia, and she conducted a survey of her own, testing those who came to her for more run-of-the-mill complaints like malaria and TB. Of the first 100, 93 were HIV positive.
Last autumn, Sister Leonia opened the only hospice in sub-Saharan Africa exclusively for Aids patients. In their homes, she found, they were often left to die without dignity. Most of the families she works with are poor and large - sometimes with 10 or 12 children - so "when someone is very, very sick they are just left in a corner of the house. The priority is given to those who are still alive."
She is one of those rare people whose goodness both humbles and inspires. I desperately wanted not to ask her the obvious awkward question; what did she think of Roman Catholic teaching on the use of condoms? Loyal servant of the church that she is, she took a deep breath before replying: "We don't promote condoms in the Catholic Church, but only someone who is very far away from the problem could say 'no, never'. If you can't control yourself it is much better for you and the person you are going to meet to use a condom." She added a touching and telling personal postscript to this open defiance of Catholic teaching: "It is my opinion, and I hope God will forgive me for it."
The invitation to produce a television series and a book on the modern Catholic Church offered both an opportunity and a challenge; an opportunity because it allowed me to discover what the much-used phrase "the universal church" really means, and a challenge because it forced me to confront the question of whether my religion's claim to Absolute Truth would stand up to objective scrutiny.
I set out on this journey armed with mildly conservative prejudices and a serviceable mental road map of the way the Catholic Church works. Eighteen months and a dozen countries on four continents later, I find myself with a special admiration for those like Sister Leonia who can be described as "loyal rebels"; people with a deep love of the Catholic faith who dare to question the church when their experience tells them it is wrong.
October l6 will be the 20th anniversary of John Paul II's election to the papacy, and the milestone will be widely celebrated. But another, less joyful anniversary has passed this summer almost without notice. It is 30 years since Pope Paul VI published the encyclical Humanae Vitae, in which he re-asserted the Catholic ban on artificial contraception. And the decision, which means that Sister Leonia is an unlikely heretic, has become a symbol of the wider problems the church has faced as it tries to adapt to today's pluralist societies.
It was arguably the most catastrophic decision by any pope this century. We pieced together the events leading up to it by talking to some of those directly involved. Two of them, John Marshall, a doctor living in south west London, and Patty Crowley, a Chicago grandmother, qualify as "loyal rebels".
They both served on the Birth Control Commission set up by the Pope in 1963 to re-examine the church's teaching on the issue. It was a time of Catholic optimism, when the second Vatican Council was putting the church through a process of much-needed reform. The way the Birth Control Commission went about its work seemed a model for the new church the council set out to create. Initially it was made up of male theologians and experts like Dr Marshall, who had done a study of natural birth-control methods. But they soon realised that, as John Marshall puts it, "the man and woman in the street - or rather the man and woman in bed - should have a say in the matter."
With papal blessing, non-specialist lay people - Patty Crowley and her husband Pat among them - were co-opted on to the commission; it was the first time in the church's history that ordinary Catholics had played a part in determining the church's mind on such a momentous question. The Crowleys were deeply involved in the Christian Family Movement, and the commission asked them to use this worldwide network of Catholic couples to conduct a survey of grassroots opinion of "the rhythm method", the approved natural form of birth control which became known as "Vatican roulette".
Three thousand couples from 18 countries responded to the Crowley's survey. From all over the world a litany of misery poured into the Spanish College in Rome where the commission was meeting, as Catholics unburdened themselves of the anguish they went through trying to reconcile church teaching with the desire to enjoy a full marriage. "The pain in those letters was heart- rending," says Patty, "we just couldn't imagine that the Church could expect such sacrifice." There were one or two sharp comments directed at the hierarchy too: "just let any priest or bishop who advocates rhythm take his rectal temperature for a few weeks," wrote a couple with six children.
The Crowley survey had a profound impact on the commission. By the summer of 1966 a majority of its members were convinced of the need for change, and that is what they recommended to Pope Paul in their final report. As the Crowleys flew back to Chicago and John Marshall made his way home to London, they and everyone else assumed it was a done deal.
Pope Paul agonised for two years before publishing Humanae Vitae in July 1968. He had encouraged the commission at every stage, which made his rejection of its conclusions all the more devastating. It was a reversal of the very ideal of church democracy which the commission had nourished.
Paul had been "got at" by a good old-fashioned Vatican plot; the conservative theologians in Rome waited until the members of the Birth Control Commission had left and then formed a new, secret commission of their own with the express purpose of undoing the work of the first. But the real significance of what happened lies in the nature of the case they used to sway Paul. It was not really about birth control, it was about authority. The logic was straightforward; for centuries the church had taught that procreation was the purpose of sex; the Holy Spirit protects the church from error, and could not therefore have allowed the church to be wrong for so long.
It is not surprising the 30th anniversary of the encyclical was allowed to pass in discreet silence. The immediate impact was to provoke something close to a revolution in the church, with walk-outs at Mass and public protests from many priests as well as lay people. The longer term impact has been much more corrosive, and is a phenomenon unprecedented in the church's history. Millions simply ignore the official teaching while continuing to regard themselves as good Catholics. A decision taken to re-assert the authority of the church has had the perverse effect of damaging it forever.
We found the problem Pope Paul faced when he reached his decision on contraception reflected in different ways almost everywhere that we filmed; how does an institution which cannot admit that it has been wrong ever change to adapt to the modern world? In Africa, Asia and Latin America we met Catholics struggling to bring the Christian message to the realities they faced, but were hamstrung by the church's determination to preserve the certainties it believes it has inherited.
Last year the Pontifical Council on the Family issued guidance for priests which effectively told them to ignore the issue of contraception in the confessional. In many parts of the church priests and even bishops quietly ignore the existence of Humanae Vitae, and the overwhelming majority of Catholics behave as if it had never been written. But, however unloved it may be, the encyclical is still there at 30, and it is an ever-present reminder of a dilemma the church cannot escape.
Edward Stourton's series, 'Absolute Truth', begins tonight on BBC2 at 8pm. The book is published by Viking.Reuse content